The present invention relates to a welding condition monitoring device capable of visually monitoring a welding state of a welding work portion of an object being weld by a welding machine by an image taken therefrom by an image sensor.
Generally, the welding operation of a modern laser welding machine is monitored by an image taken from a currently welding position by an image sensor, which image is used for inspecting the welding conditions.
FIG. 29 shows an image of a surface of members being joined together by the heat of a laser beam of a laser welder. The image is comprised of a welding portion of metal melted at high temperature by the heat of a laser beam, a molten pool and a bead formed by solidification of weld metal behind the molten pool. To estimate the quality of a weld joint to be formed, it is necessary to monitor a high luminance welding portion and a low luminance bead portion on the same image. For this purpose, it is necessary to use an image sensor having a wide dynamic range for luminosity. If an image sensor having a narrow dynamic range is applied for the above-described application, it cannot present a complete image distinctly showing both of a high luminance welding portion and a low luminance bead portion. In other words, the image taken by the image sensor having an insufficient dynamic range shows a clear bright welding work portion with an invisibly darkened bead portion or a clear bead portion with an unclear bright welding portion with halation.
Japanese Laying-Open Patent Publication No. 2000-329616 discloses a CMOS-type image sensor having a logarithmic output characteristic for attaining a wide dynamic range, which uses a matrix of light sensor circuits each of which represents a unit pixel and, as shown in FIG. 2, comprises a photodiode PD for producing therein a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 for converting the sensor current produced in the photodiode PD into a voltage signal Vpd having a logarithmic characteristic in a weak inverse state, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting the amplified voltage signal at a timing pulse Vs generated by a reading-out signal.
In the above-described image sensor, the sensor circuit, as shown in FIG. 3, may present a logarithmic output characteristic with a sufficient sensor current corresponding to the quantity of incident light falling thereon but it presents a linear (non-logarithmic) output characteristic with a small sensor current because of a delay in responding to charging/discharging of the parasitic capacity of the photodiode PD.
In case of monitoring a welding state of a welding work portion by an image taken therefrom by an image sensor using light sensor circuits each having a logarithmic output characteristic with a wide dynamic range, the image covering a wide range of luminosity from a very light portion to a dark portion may be observed but may suffer insufficient contrast of the image because of logarithmic compression of the luminance.
A recent laser welding machine is provided with a monitoring device for observing the welding state of a welding work portion on a work being welded thereon by an image taken by a camera (image sensor) to check and control the current welding conditions.
In FIG. 29, there is shown a state of a welding work portion of metals being joined together by heat of a laser beam moving in the direction indicated by an arrow, which portion comprises a molten pool BA of metal melted at a very high temperature by heat of a laser beam and a bead portion BB formed by solidification of molten metal behind the molten pool. In FIG. 29, BS designates a welding position currently being irradiated by a laser beam spot.
To visually estimate the quality of welding work, it is necessary to take by a camera a sequence of images each showing both a high luminance molten pool BA and a low luminance bead portion BB of metals being joined together by laser welding and display each image on the same monitor screen.
If a CCD camera having a narrow dynamic range is used to take an image of a welding work portion, it may present an image of the object with a clear light molten pool BA and an invisibly darkened bead portion BB (at a large diaphragm value suitable for taking a highlight portion image) or a clear bead portion and a molten pool unclear with halation (at a small diaphragm value suitable for taking a dark portion image).
Accordingly, an object image taken by a single CCD camera does not allow the observer to grasp the welding states of the molten pool BA and the bead portion BB. When a CCD camera is used for taking an image of the same object by alternately changing over the aperture size (or filter) from one suitable for taking an image of the light molten pool BA to another suitable for taking an image of the dark bead portion BB and reverse, it cannot achieve real-time monitoring of the states of the molten pool BA and the bead portion BB. Therefore, the conventional method takes at the same time two images 11 and 12 of a molten pool BA and a bead portion respectively, as shown in FIG. 34, by using two sets of CCD cameras and combines two images to present a real-time view of the object on a monitor screen. In FIG. 34, H indicates the halation and f1 and f2 indicate surface defects of the bead portion BB respectively.
When a plurality of cameras having a narrow dynamic range are used for separately taking an image of a highlight portion and an image of a relatively dark bead portion in the welding work portion of an object being welded by a welding machine such as a laser welder, arc welder and electron beam welder, each of the cameras requires separate positioning to take a specified portion following the welding process, complicating the monitoring system.
In the case of making a decision on the welding condition of a welding work portion by analyzing data obtained from the respective images taken by plural cameras, it is necessary to perform complicated processing of the image data.
Generally, the operation of a welding machine working on a transfer line for automatically welding respective works to be successively transferred to a specified position thereon shall be monitored by views taken by cameras and optimally controlled so as to ensure the sufficient quality of the weld products.
In this instance, when using the welding machine to conduct butt-welding or lap-welding of respective works to be successively transferred to the welding station on the line, it is necessary to monitor whether the welding machine always maintains its welding head at a constant distance from the work set on the welding machine based on data of images taken by the cameras. This condition is requisite for obtaining the quality of the weld product.
If a CCD camera having a narrow dynamic range is used in that case for monitoring the welding work portion, it may be adjusted by enlarging its aperture to obtain a clear image of a very light portion of the object being welded or by reducing its aperture to obtain a clear image of a relatively dark portion.
The above adjustment may result in appearing on the image a darkened invisible portion corresponding to a portion irradiated by a weakened laser beam in the teaching stage for positioning a laser beam thereon before the welding operation or may result in halation of the highlight portion on the image.
Accordingly, images clearly showing both of a low luminance portion and a high luminance portion in the actual welding process may be taken by necessarily changing over the aperture size of the camera or exchanging the filter thereof one for another.
In monitoring whether a constant distance of a welding head from a work being welded is maintained on a welding machine such as a laser welder, arc welder and electronic beam welder, the conventional monitoring method using a camera having a narrow dynamic range requires frequent changing-over of the aperture size or exchanging filters of the camera so as to present sequence of images of the work being welded, which are clear in both the low luminance portion (irradiated by a weak laser beam in a teaching stage before welding) and the high luminance portion (irradiated by an intense laser beam in the actual welding process). This requires the monitoring system to perform complicated operations.